1. Field of the Invention
The present invention relates to a water treatment apparatus employing granular activated carbon beds for purification of water.
2. Description of the Prior Art
Commonly, surface water such as lake or river water, or subterranean water, is treated in a municipal water treatment plant for use as potable water. This water often contains materials which can cause a bad taste or odor, or is otherwise harmful. For example, the water may contain organic substances from decaying vegetation, or chemicals from various agricultural or industrial applications, such as pesticides and herbicides.
It is common for waste water and potable water treatment plants to subject the water to a number of pretreatment steps. Following pretreatment, the pretreated water is filtered in one or more granular coal or sand filter beds.
As one pretreatment step, water is mixed with chemicals to coagulate particles in the water. This chemically treated water may be fed directly to the downstream filter beds. However, especially when the water has a heavy solids content, the filter beds tend to clog relatively quickly. As a result, significant amounts of backwash water is used to frequently clean the filter beds. Also while being cleaned, the filter beds may be out of service. Thus, excessive cleaning reduces the overall efficiency of the treatment plant because of the filter downtime.
To partially overcome these problems, chemically treated water is sometimes subjected to additional pretreatment steps prior to filtration. For example, a water and coagulant chemical mixture may be passed through a flocculation zone. In such a zone, gentle mixing takes place so as to promote flocculation of the solids without undue solids dispersion. From the flocculation zone, the partially pretreated water may be directed to a sedimentation basin. Solids settle in such a basin prior to final filtration. With these additional steps, the rate of plugging and head loss development in downstream filter beds is reduced. Also, cleaning water requirements are reduced. However, these added steps increase the costs of a treatment plant.
Sometimes water treatment plants, especially potable water plants, utilize additional water purification stages following an initial filter bed filtration stage. For example, filtered purified water is sometimes contacted with granular activated carbon in a fixed bed for final water polishing or purification. These carbon beds have been operated in an upflow mode, but are typically operated in a downflow mode to minimize the passage of carbon fines from the bed and into the finished water. As the water flows through these carbon beds, dissolved organic contaminants are adsorbed from the water. Eventually, the carbon becomes spent. That is, it ceases to effectively adsorb contaminants. When this happens, spent carbon is typically removed from the bed and thermally treated to regenerate it. Regenerated or previously unused fresh carbon is introduced into the bed to replace the removed spent carbon. These beds are relatively expensive in the first instance. Moreover, the active life of the granular carbon making up such beds is relatively short. Furthermore, prior art carbon beds known to applicant have suffered from the drawback of becoming quickly fouled unless the water being treated is very clear and free from suspended matter.
An exemplary activated carbon filter operated in an upflow mode, is shown in U.S. Pat. No. 4,202,770 of Gappa, et al. In the filter of Gappa, activated carbon moves downwardly through the filter in a direction countercurrent to the direction of the flow of waste water being treated. This migration is accomplished by removing contaminant laden spent carbon from the bottom of the filter, regenerating the carbon, and introducing fresh or regenerated carbon at the top of the filter bed. Like other known granulated activated carbon filters, the Gappa filter is used to purify previously filtered relatively pure water.
Another prior approach to water treatment, is described in U.S. Pat. No. 3,250,899 of Rice, et al. In Rice, fine powdered activated carbon and chemicals are added to untreated water in a plant. The mixture is then directed to a filter bed of sand. The carbon adsorbs organic materials from the water. As a result, the need for a post-filtration activated carbon bed is eliminated. However, these carbon fines penetrate the downstream filter bed and contribute to it's premature clogging. Also, it is difficult to efficiently recover costly used powdered carbon for subsequent reuse.
Therefore, a need for an apparatus and method which expands the possible uses of activated carbon beds in water treatment systems, which (a) takes advantage of the adsorption characteristics of carbon; (b) eliminates the need for the use of and problems associated with powdered carbon; and (c) incorporates the potential dual usage of granular carbon beds for adsorption as well as flocculation and suspended solids capture; (d) reduces the problem of clogging of granular activated carbon filtration beds, this latter problem having led others to limit the use of such beds to post-filtration or other pure water treatment applications.